1. Field of the Invention
The present invention relates to wireless communications, and more particularly, to a method and apparatus for performing contention-based uplink transmission in a wireless communication system.
2. Related Art
Effective transmission/reception methods and utilizations have been proposed for a broadband wireless communication system to maximize efficiency of radio resources. An orthogonal frequency division multiplexing (OFDM) system capable of reducing inter-symbol interference (ISI) with a low complexity is taken into consideration as one of next generation wireless communication systems. In the OFDM, a serially input data symbol is converted into N parallel data symbols, and is then transmitted by being carried on each of separated N subcarriers. The subcarriers maintain orthogonality in a frequency dimension. Each orthogonal channel experiences mutually independent frequency selective fading, and an interval of a transmitted symbol is increased, thereby minimizing inter-symbol interference.
When a system uses the OFDM as a modulation scheme, orthogonal frequency division multiple access (OFDMA) is a multiple access scheme in which multiple access is achieved by independently providing some of available subcarriers to a plurality of users. In the OFDMA, frequency resources (i.e., subcarriers) are provided to the respective users, and the respective frequency resources do not overlap with one another in general since they are independently provided to the plurality of users. Consequently, the frequency resources are allocated to the respective users in a mutually exclusive manner. In an OFDMA system, frequency diversity for multiple users can be obtained by using frequency selective scheduling, and subcarriers can be allocated variously according to a permutation rule for the subcarriers. In addition, a spatial multiplexing scheme using multiple antennas can be used to increase efficiency of a spatial domain.
MIMO technology can be used to improve the efficiency of data transmission and reception using multiple transmission antennas and multiple reception antennas. MIMO technology may include a space frequency block code (SFBC), a space time block code (STBC), a cyclic delay diversity (CDD), a frequency switched transmit diversity (FSTD), a time switched transmit diversity (TSTD), a precoding vector switching (PVS), spatial multiplexing (SM) for implementing diversity. An MIMO channel matrix according to the number of reception antennas and the number of transmission antennas can be decomposed into a number of independent channels. Each of the independent channels is called a layer or stream. The number of layers is called a rank.
Meanwhile, a control plane (C-plane) can be classified into an idle mode and a connected mode. The idle mode is a state in which an RRC connection is not established, that is, a UE and a BS are not connected to each other. The connected mode is a state in which the RRC connection has already been established between the UE and the BS. Since the connection is established between the BS and the UE, data can be transmitted or received bi-directionally. In addition, a dormant state and an active state can be defined to minimize power consumption of the UE in the connected mode. If there is no data transmission/reception during a time greater than or equal to a specific time in the active state in which the data is actually transmitted or received, the UE transitions from the active state to the dormant state to minimize power consumption. In the dormant state, the UE needs to rapidly transition to the active state whenever data transmission is necessary. 3rd generation partnership project (3GPP) long term evolution (LTE)-Advanced (A) requires that a transition time from the dormant state to the active state is less than or equal to 10 ms.
In general, the transition from the dormant state to the active state and started by uplink transmission can be started when the UE transmits a scheduling request (SR) to the BS. However, the SR can be transmitted only in a specific designated subframe, and in this case, a transition time is also increased by a time of waiting for the specific subframe. Therefore, there is a need to perform contention-based uplink transmission in which a transition to the activate state can be achieved without SR transmission.